Polishing apparatus and method for detecting foreign matter on polishing surface

ABSTRACT

A polishing apparatus  110  which comprises a polishing tool  1  having a polishing surface, and a holder means (top ring  4 ) for holding a semiconductor wafer (a substrate) W. Polishing apparatus  110  further comprises color CCD camera  10  for taking a color image of a region on the polishing surface; image processor means  40  for determining whether or not any foreign matter exists on the polishing surface based on a condition of a color in the color image data acquired by color CCD camera  10 , and apparatus operation control section  45  which in response to the determination of image processing section  40 , stops the relative movement between semiconductor wafer W and the polishing surface and separates top ring  4  and the polishing surface from each other.

FIELD OF THE INVENTION

The present invention generally relates to a polishing apparatus, and more particularly, to a polishing apparatus capable of detecting a foreign matter on a polishing surface, which might be produced especially by a slip-out event of a substrate during a polishing process, and also to a method for detecting the foreign matter on a polishing surface.

DESCRIPTION OF THE PRIOR ART

Conventionally, a polishing apparatus has been commonly employed as an apparatus for producing a planar surface of a semiconductor substrate. This type of polishing apparatus has a configuration in which a substrate held by a top ring (a substrate holder means) is pressed against a polishing surface of a polishing pad mounted on a top surface of a turntable and then they are slidably moved relative to each other while supplying a slurry containing abrasive grains onto the polishing surface, thus to polish a surface to be polished of the substrate.

In the polishing apparatus having the above configuration, however, sometimes it happens that the substrate slips or jumps out from the top ring during polishing. In such an event, if the polishing operation is still continued without taking an appropriate remedying action, not only would the slipped-out substrate break, but also the polishing apparatus. More disadvantageously, in the case of the break of the substrate, the removal of the broken pieces of the substrate and the re-conditioning of the polishing pad may be required before restarting the polishing process; leading to a significantly low rate of productivity.

In order to deal with such a situation, a camera is used for taking an image of the polishing surface of the polishing pad and the acquired image data is processed by which it is possible to detect a slip-out event of the substrate, or the existence of any foreign matter on the polishing pad, so that if either of the two conditions is detected, the polishing process may be suspended.

However, the conventionally available camera employed for the above-mentioned purpose is a monochrome camera, with which the detection of the foreign matter can not be ensured in a case, for example, when a color of the polishing surface and a color of the foreign matter are different from each other but both have similar brightness, thus resembling in tone and lacking in contrast. Especially for the polishing pad having the polishing surface of a dark color (e.g., a black polishing surface), it has been difficult to detect a slipped out substrate that might be a semiconductor substrate.

SUMMARY OF THE INVENTION

The present invention has been made in the light of the problems as pointed above, and an object thereof is to provide a polishing apparatus capable of detecting an existence of any foreign matter on a polishing surface in a more reliable manner and a method thereof.

In order to solve the above problems, according to an aspect of the present invention, a polishing apparatus comprising a polishing surface and a substrate holder means for holding a substrate and pressing the substrate against the polishing surface with a surface to be polished of the substrate brought into contact with the polishing surface, in which the substrate can be polished by a relative movement between the substrate and the polishing surface, is provided. The polishing apparatus further comprises a color camera for taking an image of a region on the polishing surface defined in the vicinity of the substrate holder means, and an image processing section for determining whether or not a foreign matter exists on the polishing surface based on a condition of a color in a set of image data acquired by the color camera.

According to another aspect of the present invention, a polishing apparatus characterized in that the image processing section comprises: an identifying means for identifying whether or not a color of each point in the image data represents a color of a foreign matter; and a determination means which determines that the foreign matter exists if a total area of those points having the colors screened and identified to represent the foreign matter is larger than a predetermined threshold value are provided.

According to another aspect of the present invention, a polishing apparatus characterized in that the image processing section comprises: a screening means for screening to identify whether or not a color of each point in the set of image data is identical with a color of a foreign matter, which has been previously stored as a reference color, or identical with a color representative of the polishing surface; and a determination means which determines that the foreign matter exists if either an area corresponding to the reference color or an area not corresponding to the reference color goes beyond a corresponding predetermined threshold value are provided.

According to still another aspect of the present invention, a polishing apparatus is provided, which is characterized in further comprising an apparatus operation control section, which in response to the determination of the image processing section that the foreign matter exists, stops the relative movement between the substrate and the polishing surface and separates the substrate holder means and the polishing surface from each other.

According to another aspect of the present invention, a detection method is provided, for detecting a foreign matter on a polishing surface during a polishing process where a substrate is being polished by making a relative movement between the substrate and the polishing surface while pressing the substrate against the polishing surface, the method comprising the steps of: taking an image of a predetermined region on the polishing surface by using a color camera; screening to identify whether or not a color of each point in a set of image data taken by the color camera is identical with a color of a foreign matter, which has been previously stored as a reference color, or identical with a color representative of the polishing surface; and determining that the foreign matter exists if either an area corresponding to the reference color or an area not corresponding to the reference color goes beyond a corresponding predetermined threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view illustrating main components of a polishing apparatus 110.

FIG. 2 is a general flow diagram showing an example of a foreign matter detection method.

FIG. 3(a) and FIG. 3(b) show polishing surfaces of a polishing tool 1, respectively, represented in two-color pattern.

FIG. 4 is a schematic diagram illustrating a specific method for detecting a foreign matter on the polishing surface.

FIG. 5 is a general schematic view illustrating an example of configuration of a polishing apparatus equipped with a cleaning unit.

In the drawings, reference numeral 110 (110 a, 110 b) designates a polishing apparatus, 1 a polishing tool, 2 a turntable (polishing table), 3 a table turning shaft, 4 a top ring (substrate holder means), 5 a top ring turning shaft, 6 a top ring swing arm, 7 a swing arm turning shaft, 10 a color CCD camera (a color camera), 40 an image processing section, 45 an apparatus operation control section, 50 an abrasive liquid supply pipe, symbol “S” an abrasive liquid (a slurry) and “W” a semiconductor wafer (a substrate), respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary chemical and mechanical polishing plant (CMP plant) equipped with a polishing apparatus and a cleaning unit will now be described, which represents one application of the present invention.

FIG. 5 is a general schematic view illustrating an example of a polishing plant of this type. The polishing plant 110 comprises, as shown in FIG. 5, a pair of two similarly configured polishing apparatuses 110 a, 110 b which are located symmetrically in the left and the right sides of the plant. A cleaning unit 126 includes a pair of two primary cleaning machines 126 a 1, 126 a 2, a pair of two secondary cleaning machines 126 b 1, 126 b 2, and a pair of two turn-over machines 128 a 1, 128 a 2, respectively, which are located symmetrically in the left and the right sides corresponding to respective polishing apparatuses 110 a, 110 b, and further includes two transfer equipments 124 a, 124 b. In addition, two load and unload stations 122, 122 are arranged symmetrically in the left and the right sides, respectively.

The polishing apparatus 110 a, 110 b comprises a turntable (a polishing table) 2 a, 2 b and a top ring 4 a, 4 b for pressing a semiconductor wafer held on an under surface thereof against the turntable 2 a, 2 b so as to polish the wafer.

In the polishing apparatus having such a configuration as described above, the semiconductor wafer is transferred by the transfer equipments 124 a, 124 b from the load and unload station 122 to a delivery table 138 a (or 138 b) to be sucked there onto the under surface of the top ring 4 a (or 4 b), which is in turn moved to a position above the turntable 2 a (or 2 b). A polishing tool 1 a, 1 b, such as a polishing pad or a bonded abrasive having a polishing surface formed on the top surface thereof, is mounted on a top of the turntable 2 a, 2 b. Then, while supplying a specified abrasive liquid (in specific, for polishing an insulation film (an oxide film) on a silicon wafer, the abrasive liquid of alkaline aqueous solution with abrasive grain particles having a specified particle size suspended therein), and also while rotating the turntable 2 a (or 2 b) and the top ring 4 a (or 4 b) respectively, the semiconductor wafer is pressed against the polishing surface thus to polish the semiconductor wafer. After having been finished with the polishing process, the semiconductor wafer is passed through a cleaning and a drying process and delivered back to the load and unload station 122.

The primary cleaning machine 126 a 1, 126 a 2 is a low speed rotary type cleaning machine in which a plurality of vertical rollers 130 is arranged surrounding the wafer, and the wafer is held at an outer periphery thereof by grooves formed on the upper peripheries of the rollers 130 so that the wafer may be driven to rotate by the rotation of the rollers 130, wherein cleaning members made of sponge in the form of a roller or a pencil are provided so as to come into contact with and to be retracted from the wafer from the above and the below directions. The secondary cleaning machine 126 b 1, 126 b 2 is a high speed rotary type cleaning machine having a wafer gripping arm extending radially from a top end of the turning shaft.

After the above-discussed polishing process, a cleaning process is carried out in the following manner. Firstly, in the primary cleaning machine 126 a 1 (or 126 a 2), the wafer is subjected to a scrub cleaning in which the wafer is scrubbed with the cleaning polishing members to be cleaned up while being rotated and also supplied with a cleaning liquid over the top and the bottom surfaces thereof.

Then, in the secondary cleaning machine 126 b 1 (or 126 b 2), the wafer is further cleaned and subsequently subjected to a drying process by a high speed spinning of the secondary cleaning machine. After having finished with the cleaning and the drying processes, the wafer is returned to the load and unload station 122 by a clean hand of the transfer equipment 124 b.

Two different types of operation may be carried out selectively in the above described facility of polishing apparatus: one is a parallel operation in which two polishing apparatuses 110 a, 110 b provide polishing process independently from each other for the wafers supplied thereto respectively, and the other is a serial operation in which a single wafer is transferred through the two polishing apparatuses 110 a, 110 b sequentially so as to be subjected to different polishing processes therein respectively.

In the parallel operation, in which each of the polishing apparatuses 110 a, 110 b functions independently for providing both a regular polishing and a finishing polishing, a water polishing operation in which only the water is supplied rather than the abrasive liquid may be carried out at different timings between the respective polishing apparatuses 110 a, 110 b so that the transfer equipments 124 a, 124 b may transfer the semiconductor wafer in an efficient manner.

Since this polishing facility comprises two polishing apparatuses 110 a and 110 b as well as the primary and the secondary cleaning machines 126 a 1, 126 a 2, 126 b 1 and 126 b 2, two separate wafer processing lines may be established: one is a first wafer processing line providing the sequential steps comprising a polishing process by using the polishing apparatus 110 a, a primary cleaning process by using the primary cleaning machine 126 a 1 and a secondary cleaning process by using the secondary cleaning machine 126 b 1, and the other is a second wafer processing line providing sequential steps comprising a polishing process by using the polishing apparatus 110 b, a primary cleaning process by using the primary cleaning machine 126 a 2 and a secondary cleaning process by using the secondary cleaning machine 126 b 2, and therefore the semiconductor wafer transfer lines can be operated independently without interfering with each other, thus improving the efficiency of the cleaning operation.

In the serial operation, after the regular polishing has been applied to a semiconductor wafer by polishing apparatus 110 a, the semiconductor wafer is transferred to the polishing apparatus 110 b, where the water polishing operation is applied to the wafer. If there is no problem of contamination on the polishing apparatus, the semiconductor wafer may be transferred directly from polishing apparatus 110 a to polishing apparatus 110 b by transfer equipment 124 a. If there is a problem of contamination, after the regular polishing having been applied to the semiconductor wafer in the polishing apparatus 110 a, the semiconductor wafer should be transferred to primary cleaning machine 126 a 1 by transfer equipment 124 a to be cleaned up therein, and subsequently transferred to polishing apparatus 110 b, where in turn the water polishing operation is applied to the semiconductor wafer.

In this case, any preferred chemicals may be selected depending on the type of slurry used in polishing apparatus 110 a, so as to be added during the cleaning process in primary cleaning machine 126 a 1. In this serial operation, since the regular polishing operation and the water polishing operation are independently performed on separate turn tables 2 a, 2 b, respectively, changing the polishing liquid to be supplied onto the turntable from the abrasive liquid to the purified water and vice versa, is not necessary, thus preventing the increase of loss time in operation as well as the increase of consumption of the abrasive liquid and purified water.

The present invention further comprises a foreign matter detection means arranged in the above-described polishing apparatus 110 (110 a, 110 b), for detecting whether or not the foreign matter exists on the polishing surface due to the slip-out of a semiconductor wafer (a substrate) while being polished.

FIG. 1 is a schematic front view illustrating main components of polishing apparatus 110. As shown in FIG. 1, polishing apparatus 110 comprises a turntable (a polishing table) 2, a top ring (a substrate holder means) 4, a color CCD camera 10, an image processing section 40 for processing a set of image data acquired by the camera, and an apparatus operation control section 45 for controlling an overall operation of the polishing apparatus 110. Each of the components will be described below.

The turntable 2 is in a disc-like shape and has a table turning shaft 3 mounted in a central location on a lower surface thereof and further a turntable driving section 15 below the table turning shaft 3, which drives the turntable 2 to rotate via the table turning shaft 3. A polishing tool 1 formed by, for example, a polishing pad or a bonded abrasive (abrasive grains bonded by using a resin binder) is mounted on a top surface of the turntable 2.

The top ring 4 has a top ring turning shaft 5 mounted in a central location on a top surface thereof with an upper portion of the top ring turning shaft 5 inserted into a top ring swing arm 6, so that the top ring 4 may be driven to rotate and/or to move up and down by a top ring rotary driving means 61 and a top ring vertical driving means 63, each arranged on the top ring swing arm 6. The top ring swing arm 6 is designed to be swung by a swing arm turning shaft 7. This means that the top ring 4 is operatively designed so as to move freely between a delivery table 138 (a, b) and the turntable 2 (a, b) shown in FIG. 5 with the aid of the swing arm turning shaft 7. In addition, an abrasive liquid supply pipe 50 for supplying an abrasive liquid (a slurry) S is arranged above the turntable 2.

The color CCD camera 10 is attached on a side wall of the top ring swing arm 6 by an arm 11 so as to be positioned in the vicinity of a side portion of the above-described top ring 4. Due to this arrangement, color CCD camera 10 is able to take an image of a region on the polishing surface of turntable 2 in the vicinity of top ring 4 during the polishing process. Preferably, color CCD camera 10 may be installed in the specified position above turntable 2 located downstream with respect to the rotation thereof, where the semiconductor wafer W is more likely to slip out. If color CCD camera 10 is fixedly attached to top ring swing arm 6 in a manner discussed above so as to be swung therewith, top ring swing arm 6 serving as a swing motion mechanism for top ring 4, then even in such a case that the polishing is carried out while swinging top ring 4, advantageously the image-taking position of color CCD camera 10 may be normally fixed with respect to top ring 4. It is a matter of course that color CCD camera 10 may be operatively mounted to other mounting means, such as an arm independently arranged separately from the top ring 4, so that this mounting means may be swung to position color CCD camera 10 in the vicinity of the side portion of top ring 4.

Image processing section 40 is designed so as to receive the image data of the polishing surface acquired by color CCD camera 10, to determine whether or not any foreign matter exist in the region acquired as the image, and then to output the result of the determination to apparatus operation control section 45.

Apparatus operation control section 45 controls the overall operation of polishing apparatus 110, and specifically it provides an independent control of the number of revolutions for turntable 2 and top ring 4 and in addition it also controls a pressure force of semiconductor wafer W against the polishing surface by moving top ring 4 up or down, a swing motion of top ring swing arm 6 and/or an amount of supply of slurry S.

Next, a detection method of foreign matter on a polishing surface during a polishing process of a substrate by polishing apparatus 110 will be described in detail.

FIG. 2 is a general flow diagram showing a method for detecting a slip-out event of semiconductor wafer W during the polishing process by using above-described color CCD camera 10 or the like. Semiconductor wafer W held on the under surface of top ring 4 in a manner as described above is brought into contact with the polishing surface of polishing tool 1 and polished by the rotating motions of top ring 4 and turntable 2, while in this period, image processing section 40 receives the images acquired by color CCD camera 10 at a rate of some ten times to some hundred times per second (Step 1), and determines whether or not any foreign matter, typically a part of semiconductor wafer W which has slipped out of the top ring 4, exists on the polishing surface based on a specified determination method (Step 2).

Specifically, the determination method includes, for example, the following methods:

(Determination Method 1)

First of all, a color representative of semiconductor wafer W, which will be determined as the foreign matter, should have been input and stored in advance as a reference color. Image processing section 40 compares the color of each point in the image of the image data received from color CCD camera 10 with the reference color individually so as to identify that the color of the point represents the color of the foreign matter or the color of the polishing surface. Then, in the image at a certain moment, when an area of points (a surface area formed by the points) identified to represent the color of the foreign matter has extended to be ultimately larger than a previously determined specific area (a threshold value), image processing section 40 determines that the foreign matter exists on the polishing surface.

That is, as shown in FIG. 4 by way of example, in the image data at a certain moment received from color CCD camera 10, if the area of the points screened out to be identified as the foreign matter (the area painted into black) is not larger than the predetermined specific area (the threshold value) as represented by image P1, then image processing section 40 determines that there is no foreign matter. On the other hand, if the area of the points screened out to be identified as the foreign matter (the area painted into black) has extended to be finally larger than the predetermined specific area (the threshold value) as represented by image P2, then image processing section 40 determines that the foreign matter exists on the polishing surface.

If the above-described specific area is set to be small, then the detection sensitivity will be enhanced but there may be a possibility of an erroneous detection due to wrong identification. The optimal set area may be varied depending on a range of the image acquired by color CCD camera 10, a size of semiconductor wafer W, and a relationship between the frequency of the image processing and the number of revolution of turntable 2, and preferably the set specific area should be around one half of the total area of semiconductor wafer W. Further, preferably the reference color should be set to have a width of color (a certain range of wave length) rather than a single color, and so a more stable screening and determination can be provided.

Although in the above-discussed determination method, it is determined that the foreign matter exists on the polishing surface when the area of the points screened out to be identified as the foreign matter has extended over the predetermined threshold value, the area of the foreign matter should not be necessarily a criterion, but alternatively image processing section 40 may determine that the foreign mater exists when the area, which has not been screened out as the foreign matter, has been reduced to be ultimately smaller than a predetermined area (a threshold value).

(Determination Method 2)

The color of the semiconductor wafer representing the foreign matter has been set and stored in determination method 1, but determination method 2 employs instead a color of the polishing surface to be set and stored as the reference color. In this case also, image processing section 40 compares the color of each one of the points making up the image of the image data received from color CCD camera 10 with the reference color individually, and in this comparison, image processing section 40 determines the point having the color from among the reference color to be the foreign matter.

Then, similarly to determination method 1, in the image at a certain moment, when the area of the points screened out to be identified as the foreign matter has extended to be ultimately larger than the predetermined specific area (the threshold value), image processing section 40 determines that the foreign matter exists on the polishing surface. Preferably, the reference color should be set to cover a certain range of color. The area of the foreign matter should not necessarily be the criterion, but alternatively image processing section 40 may determine that the foreign matter exists when the area, which has not been screened out as the foreign matter, has been reduced to be ultimately smaller than the predetermined area (the threshold value).

It is to be appreciated that since generally slurry S is being supplied during the polishing of the substrate and this may change the color of the polishing surface, therefore in this determination method, the reference color should be set also by taking the color of the slurry S (the color of the polishing surface changed by the supplied slurry S) into consideration. There will be also a case where the color of the polishing surface is changed when slurry S is replaced by purified water for performing, what is called, the water polishing or when slurry S is changed from one type to another during the polishing, depending on polishing process applied.

In either case, each of the different colors of the polishing surface generated by the supply of respective different slurries S should have been set in advance as the reference color, and in the case of starting, switching or stopping of the supply of slurry S under the control of apparatus operation control section 45, apparatus operation control section 45 may output a signal indicative of the operation to image processing section 40 so as to switch the reference color from one color to another to be used in the determination by image processing section 40 so that image processing section 40 may make a correct determination on whether or not the foreign matter exists on the basis of the newly changed reference color. This may enable a stable detection of the foreign matter.

(Determination Method 3)

Determination method 3 employs polishing tool 1 having the polishing surface patterned with two different colors. For example, the color of the polishing surface of polishing tool 1 may have a color pattern of radial lines in two different colors consisting of bright color areas al and dark color areas a2 arranged alternately as shown in FIG. 3(a), or a color pattern of check pattern as shown in FIG. 3(b). Either pattern is represented by black and white in the drawings, but preferably the actual colors should be chromatic colors. Each element of the above-described patterns should be made small enough in comparison with the range of the image taken by color CCD camera 10 so that the ratio of the total area occupied by one color in the image acquired during the rotation of turntable 2 to that occupied by the other color may change a little or may be approximately constant. Alternatively, a pattern formed to be parallel with the proceeding direction of the polishing surface, or a coaxial circular pattern for the turntable, may be employed to eliminate substantially the change in ratio of one color to the other color otherwise caused by the movement of the polishing surface.

These two colors in the pattern should have been set and stored in advance as the reference colors in image processing section 40, and a total area of each one of the reference colors occupying the image at a certain moment is determined respectively. In determination on whether or not a foreign matter exists, it is determined that the foreign matter exists when either one of the areas of two different colors has fallen out of the range of the change in area due to the rotation of turntable 2 to be smaller than the specified area. If the color of the polishing surface is similar to the color of the foreign matter, there will be a possibility that the determination on whether or not the foreign matter exists is uncertain, but according to this method using two different reference colors, the foreign matter of any color would be apparently different from at least either one of the two different reference colors and so the detection can be performed with higher reliability. In this case also, preferably each reference color should be set to cover a certain range of color.

Three different determination methods have been described as the embodiments of the present invention, and since all three methods according to the present invention employ a color camera as the image taking means, each one of the points in the image acquired by the color camera contains individual set of gradient data for each one of three primary colors. Owing to this, each of the gradient data may be compared individually and thereby a difference in color pertaining to an object can be detected, which could not have been detected through the comparison of brightness in a black and white image or a monotone monochrome image.

Referring again to FIG. 2, when the image processing section 40 has determined that no foreign matter exists according to one of the above-described determination methods, the operation of polishing apparatus 110 is continued and the above-discussed determination process (Step 1 and Step 2) may be sequentially repeated.

On the contrary, when image processing section 40 has determined that foreign matter exists according to one of the above-described determination methods, a signal indicative of that determination is sent from image processing section 40 to apparatus operation control section 45, which in response to this, stops immediately the polishing operation in order to prevent damage to semiconductor wafer W as well as to polishing apparatus 110. (Step 3). In specific, the rotating motions of turntable 2 and top ring 4 are stopped, and top ring 4 is lifted up to separate from polishing tool 1. Further, any alarm sounds or alarm signals may be sent to a central control room in a semiconductor manufacturing plant.

It is to be noted that as is the case with the polishing apparatus equipped with the cleaning unit shown in FIG. 5, if the facility of polishing apparatus 110 (110 a, 110 b) comprises a plurality of turntables 2 (2 a, 2 b) and a plurality of top rings 4 (4 a, 4 b), and/or comprises the built-in cleaning unit (cleaning and drying unit 126), only the operation of the polishing apparatus in concern (for example, apparatus 110 a) may be stopped but polishing apparatus (110 b), cleaning unit 126 and so on may continue their specified operations.

Although the present invention has been illustrated and described with reference to the preferred embodiments, the present invention is not limited to those embodiments, and many different variations may be made without departing from the scope of the disclosure in the claims and the technical concept described in this specification and the attached drawings. It is to be noted that other shapes and structures, which are not directly illustrated in the specification and drawings but can achieve the operation and/or the effect of the present invention, are intended to fall within the scope of the technical concept of the present invention.

For example, although polishing apparatus 110 using rotary turntable 2 has been illustrated in the above embodiments, it is needless to say that the present invention is applicable to such a polishing apparatus having a configuration in which a substrate is pressed against a polishing belt moving linearly. The point is that the present invention is applicable to a polishing apparatus of any configuration as far as it comprises a polishing surface and a substrate holder means, in which the substrate held by the substrate holder means is pressed against the polishing surface with a surface to be polished of the substrate brought into contact with the polishing surface and then the substrate and the polishing surface driven to make a relative movement to each other so as to polish the substrate.

Although the description in the above embodiment has been directed to the example for detecting semiconductor wafer W slipped out of top ring 4 as foreign matter, it is needless to say that the present invention is applicable to the detection of a variety of types of foreign matters other than the semiconductor wafer.

Although the number of colors used is two in the above determination method 3, three or more colors may be used, and in this case those three or more colors (or a specified number of colors selected among them) may be used as reference colors for the determination of foreign matter.

EFFECT OF THE INVENTION

According to the present invention, as described above in detail, since a color camera has been employed as a camera used to take an image of a polishing surface, and each one of the points in the acquired image contains a set of color gradient data for each one of three primary colors, which will be compared individually, a difference in color pertaining to an object can be detected more precisely, which could not have been achieved in the conventional comparison of contrast, and advantageously the existence of any foreign matter can be detected in a more reliable manner, thus providing a superior effect in ensuring that both of the substrate and the polishing apparatus can be protected from possible damage. 

1. A polishing apparatus comprising: a polishing surface; a substrate holder means for holding a substrate and pressing a surface to be polished of the substrate against the polishing surface, the substrate and the polishing surface being relatively moved so that the surface to be polished is polished; a color camera for acquiring color image data of a region of the polishing surface; and an image processor means for determining whether or not foreign matter exists on the polishing surface based on color condition of the color image data.
 2. A polishing apparatus according to claim 1, wherein the image processor means comprises: an identifying means for identifying whether or not a color of each point in the color image data is a color of foreign matter; and a determination means which determines existence of foreign matter when a predetermined threshold value is exceeded by a total area of points each of which color is the color of foreign matter.
 3. A polishing apparatus according to claim 1, wherein the image processor means comprises: an identifying means for identifying whether or not a color of each point in the color image data is identical with a color of a foreign matter, which has been previously stored as a reference color, or identical with a color of the polishing surface; and a determination means which determines existence of foreign matter when a total area of the points each of which color corresponds to the reference color exceeds a predetermined threshold value, or when a total area of the points each of which color does not correspond to the reference color decreases to less than a predetermined threshold value.
 4. A polishing apparatus according to claim 3, wherein the predetermined threshold value depends on a range of the color image data acquired by the color camera, a size of substrate, or a relationship between frequency of processing the color image and number of revolution of the polishing surface, and wherein the reference color has a width of color.
 5. A polishing apparatus according to claim 1, further comprising an apparatus operation control means, which in response to the decision of the image processor means that the foreign matter exists, stops the relative movement between the substrate and the polishing surface and separates the substrate holder means from the polishing surface.
 6. A polishing apparatus according to claim 1, wherein the color camera for acquiring color image data of a region of the polishing surface in the vicinity of the substrate holder means.
 7. A method for detecting a foreign matter on a polishing surface during a polishing process where a substrate is polished by a relative movement between the substrate and the polishing surface while the substrate is pressed against the polishing surface, the method comprising the steps of: taking a color image data of a predetermined region on the polishing surface by using a color camera; identifying whether or not a color of each point in a set of color image data taken by the color camera is identical with a color of a foreign matter, which has been previously stored as a reference color, or identical with a color of the polishing surface; and determining that the foreign matter exists when a total area of the points each of which color corresponds to the reference color exceeds a predetermined threshold value, or when a total area of the points each of which color does not correspond to the reference color decreases to less than a predetermined threshold value.
 8. The method according to claim 7, wherein a predetermined threshold value depends on a range of the color image data acquired by the color camera, a size of substrate, or a relationship between frequency of processing image and number of revolution of the polishing surface, and wherein the reference color has a width of color.
 9. A polishing apparatus comprising: a polishing surface; a substrate holder for holding a substrate and pressing a surface to be polished of the substrate against the polishing surface, the substrate and the polishing surface being relatively moved so that the surface to be polished is polished; a color camera for acquiring color image data of a region of the polishing surface; an image processor means for determining whether or not foreign matter exists on the polishing surface based on color condition of the color image data, and wherein the image processor means comprises: an identifying means for identifying whether or not a color of each point in the color image data is identical with a color of a foreign matter, which has been previously stored as a reference color, and a determination means which determines existence of foreign matter when a total area of the points each of which color corresponds to the reference color exceeds a predetermined threshold value.
 10. A polishing apparatus according to claim 9, further comprising a control unit for outputting a signal to switch the reference color to another color.
 11. A polishing apparatus according to claim 10, further comprising a plurality types of polishing liquid which are supplied to the polishing surface; wherein the control unit controls a supply of the plurality types of polishing liquids, and the control unit outputs the signal to switch the reference color based on the type of polishing liquid supplied to the polishing surface. 